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Messages - jrista

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1186
Reviews / Re: Why the DxO bashing?
« on: February 07, 2014, 10:04:47 PM »
[

I've been on a rampage against DXO lens tests for almost as long as they've been around, and I've been QUITE vocal about that here in these forums. DXO lens tests frequently indicate that Canon lenses are better than the competitions, DESPITE the undue bias they give to Nikon lenses thanks to the D800. Doesn't change the fact that DXO's lens tests are a joke, again thanks to that "weighted scoring" they do that vastly overweights factors that don't play a big roll in IQ, and vastly underweights factors that do play a big role. They also use the word "transmission" to refer to what is really "aperture", and therefor ALL of their lens tests are massively skewed by the transmission factor.



 
I have a D300s and a Nikon 200-400mm VRI, which is a nice lens by anyone but DXO's measurements.  They tested the VRII model which is basically the same lens with improved VR and give it a score of 12! 
http://www.dxomark.com/Lenses/Nikon/Nikon-AF-S-Nikkor-200-400mm-F4G-ED-VR-II-mounted-on-Nikon-D300s__614
 
Now, if you put it on a D7000, it jumps to 14.  On a D3 its 17, and on a D700 its also 17.  On a D4, it jumps to 21, on a D600, it jumps to 24, and on a D800, its 25!
This is why you don't compare lenses across different camera models, much less across different brands.  Testing lenses on a camera body gives results that only apply to that body or one with the same sensor.  In general, the test methods will give higher numbers with more MP.  The lens itself did not change and is no better or worse just because its on a camera with more pixels.
The reason is simple, the MTF of a image is a product of the MTF of the lens, The Body, the monitor or printer its viewed on, and even your eyes.  Raising the MTF of any of these things will improve the image as long as the others don't change.  Of course, DXO does not USE MTF,  just because the entire photography world uses it, they invented their own number, MPIX.
 
Now, if DXO wanted to compare lenses between Canon and Nikon, they'd test them all on the same Canon body.  That would give you at lease some comparison, but it still would not be accurate, since manufacturers cameras recognize a lens model and may make subtle adjustments to exposure at the edges.
 
That's why most knowledgeable lens testers provide a warning note that says don't compare on different models or manufacturers, a lens test on a D300s is only good for a D300s, but may be similar for a body with the same sensor.

Aye. All this I know. Which is why I always say: Standard lens testing is useless! It has no real value, since output resolution is usually sensor bound. These tests don't really tell you anything about the lens, and they aren't comparable...so....what value do they hold? Personally, I just made the decision to ditch lens tests entirely and rely on model-generated MTFs to determine how good a lens is. You can actually garner a LOT of useful knowledge about a lens from a single MTF chart, not the least of which is corner performance.

Anyway....

1187
Reviews / Re: Why the DxO bashing?
« on: February 07, 2014, 02:59:09 PM »
People bash DxO because during the time when DxO results have been shared around more and more widely talked about on the 'Net, they always favour Nikon/Sony.

As a result, all sorts of reasons have been created by the Canon fanbase as to why this is and how DxO is useless but rest assured, if the shoe were on the other foot, people would be lauding DxO.

LOL. Your ignoring the facts here, bub. I've been on a rampage against DXO lens tests for almost as long as they've been around, and I've been QUITE vocal about that here in these forums. DXO lens tests frequently indicate that Canon lenses are better than the competitions, DESPITE the undue bias they give to Nikon lenses thanks to the D800. Doesn't change the fact that DXO's lens tests are a joke, again thanks to that "weighted scoring" they do that vastly overweights factors that don't play a big roll in IQ, and vastly underweights factors that do play a big role. They also use the word "transmission" to refer to what is really "aperture", and therefor ALL of their lens tests are massively skewed by the transmission factor.

It doesn't matter who DXO favors. Their science is too obscure, and the results are too skewed, to take any of their scores at face value without a significant dose of salt grains. It really has nothing to do with who they cater to, and everything to do with how they do things.

1188
Reviews / Re: Why the DxO bashing?
« on: February 07, 2014, 01:25:51 PM »
In various threads around CR, I have noticed quite a few opinions that are not complimentary to the folks at DxO. The various individuals seem to take issue with DxO's methods and conclusions and generally disagree with pretty much everything they offer. Why? Is there some inherent fault with their methodology that would make their conclusions erroneous? (I am neither pro or con on this issue, but would just like some enlightenment.) Do you have any factual basis for disagreement? Comments?

It should be noted that, in addition to all the valid reasoning offered by everyone else, that DXO's scoring system is not just invalid for Canon. It is invalid, period. Because they utilize only ISO 100 measures in several scores, and because they do not FULLY publish all of their algorithms and methodologies, it invalidates the process as a whole. Using DXO to find the best Nikon camera is no better, and the results are just as misleading for them as for any other brand.

For example, Print DR is an EXTRAPOLATION based on DERIVED data based on Screen DR, which itself is determined via an unknown formula...it is not actually a "measure", despite the fact that they claim as much. Print DR is also based on the assumption that images are downsampled to a standard 3200x2400 pixel size. When it comes time to edit RAW images, ALL RAW images are ALWAYS edited at 100% size. It's the nature of RAW. So when DXO says some Nikon DSLRs are capable of achieving more than 14 stops of DR, they are being extremely misleading to people who don't understand what those numbers really mean. There is NO camera that uses 14-bit ADC that can achieve more than 14 stops of DR. Technically speaking, because of required overhead, even if that overhead is minimized, one could never really achieve 14 stops of DR period, you would always attain something just a little less.

Screen DR is the only valid measure of DR that DXO offers. In the case of the D800 and D600, the actual DR is ~13.2 stops, as much as 1.2 stops LESS than the 14.4 one might believe they have according to the Print DR extrapolation. The fact that Print DR IS an extrapolation means that even if someone downsamples their image by a considerable amount to 3200x2400 pixels, then tries to recover that extra 1.2 stops of information, it may not actually exist...Print DR is not a measure. It is derived, and therefor no one can really know for certain if a downsampled image from a D800 or D600 could actually achieve that much dynamic range. From an editing latitude standpoint...how much real-world freedom you have to push exposure around in post, you can only use DXO's Screen DR measure. The problem is, that measure is not shown by default, you have to find it yourself.

DXO claims a scientific approach to measuring cameras. They really fail to provide that on multiple fronts. For one, they don't measure cameras at all...they measure sensors, and only sensors. That fails to factor in ANY other camera features, such a AF system, metering system, body ergonomics, or other camera capabilities that may be as critical, if not more critical, to final IQ than the sensor. Because they do not 100% fully disclose all details of every methodology and algorithm they use, they can't really claim to be scientific (or at least, not openly scientific.) The fact that they clearly seem to have changed some of their algorithms over the years, and sometimes explicitly to cover up errors they made previously, only debases their claim to using a scientific method even more. At the very least, if an error is found in their methodology, it should be clearly explained and not slyly covered up.

Finally, as others have said...trying to reduce such a complex device as a camera to a single score that could theoretically be used for simple comparison of one camera to another is doomed to failure from the get-go....too many variables to consider, especially considering that DXO ignores the vast majority of them (i.e., all the non-sensor factors.) And don't even get me started on their lens tests...thats a debacle that puts their sensor testing to shame! :P

1189
Reviews / Re: Why the DxO bashing?
« on: February 07, 2014, 12:56:39 PM »
I cannot help but question the measurement methodology of a group who (either fraudulently or ignorantly) uses biased and misleading summary statistics to put forth claims about camera/lens performance.  That is to say, if you don't analyze your data properly but staunchly claim to be fair and objective, then it is my obligation to question your data collection methods as well, because your entire process is now suspect.  That is what any good scientist does.

DxO has the phrase "Image Science" as part of their logo, but their practices aren't consistent with that phrase.  I'm most concerned by the 'black box' calculation for their summary Sensor Score and Lens Score (methods should be published), and by the fact that they released data which was incorrect, defended it, then subsequently changed it with no acknowledgement of their error.  Also, I'm noticing that the more I delve into their Measurements, the more I find errors (for example, I just looked at the Canon 28-300L measurements and their actuance data shown visually as field maps are ~10% lower relative to the same data plotted on a graph as a profile).

This right here sums up my problems with DXO in a nutshell. They HAVE made mistakes in the past, ignored them, then quietly tried to fix them without a word. That's irksome. I'm not saying that their DR tests (or rather, Screen DR measurements) are wrong...they are just as valid as any other form of DR test, so long as the same method is applied with consistency.

My problem is that DXO does not clearly explain all of their methods, and sometimes their methods seem sketchy. For example, Print DR is not actually measured, it is derived. Print DR is the "measure" (as they call it) used in scoring their camera sensor tests. But it is not a measure, it is a derivation from the actual Screen DR measure. Problem with Screen DR is, they don't actually publish the actual method by which they measure it, so not only do you have Print DR with is derived from Screen DR...you don't really know how Screen DR is computed. That's quite frustrating. (It gets even worse when you download DXO Optics Pro to see how it works, and find that DXO's algorithms result in FAR more noise with Canon CR2 files than ACR/LR, RawThearapy, or any one of various astrophotography tools...makes me even that much more suspicious that DXO doesn't really know how to properly process Canon CR2 files for optimal performance.)

1190
Third Party Manufacturers / Re: Sigma to Add Second MTF Chart for Lenses
« on: February 06, 2014, 12:48:51 PM »
LOL. Sorry, but you entirely misunderstood the point of my blog article, which had to do with the myth of diffraction as it relates to pixel size, a myth that presumes once you stop a lens down to the diffraction limited resolution of the sensor, you suddenly experience worse IQ than a sensor with larger pixels (yes, many photographers actually DO believe that). That's a different issue, though.

Smaller pixels won't automatically make the result worse, except if their small size means relatively more space is dedicated to non light gathering circuitry. But the claims I saw in that blog go further: "That means softening caused by diffraction can fairly easily be corrected with some sharpening while post-processing.". It then goes on to show that F/22 and sharpening yields the same result as F/8 here, although even with this sample image the extra noise from F/22 and sharpening is quite obvious.

And the reason for this extra noise is simple to explain: the diffraction limited lens acts as a low pass filter, which unfortunately does not low pass filter sensor noise at the same time. Which means you lower the signal to noise ratio for higher image frequencies. Once you boost the higher frequencies, you also boost high frequency noise components, and that's what you see in that sample image.

Actually, the noise in the f/8 vs f/22 example is primarily due to the fact that the image was saved as an animated GIF (256 color palette). The first frame is what the color palette is based on, all subsequent frames kind of get the shaft when it comes to their color, so they end up a little more noisy (the exactly correct colors for the f/22 image cannot be found in the color palette, so the nearest color is picked instead). You have to realize there was a pretty minimal mount of sharpening involved there...not enough to produce artifacts or enhance noise to the point it is a visible problem.

As for sharpening, it mitigates the impact of diffraction, it does not eliminate the effects of diffraction entirely, or make lenses behave purely geometrically. Sharpening an f/22 image does not make it diffraction limited f/2 performance. There are also limits as to how far sharpening takes you the farther you stop down...sharpening an f/32 or f/45 or f/64 is certainly not going to reduce the impact of diffraction enough to produce geometric results. It does, however reduce the muddiness of diffraction blurring that affects the f/16+ image to an acceptable level. But that's all post-processing. Lenses behave as lenses behave. Anything you do in post does not actually change the behavior of the lens.
The diameter of an Airy Disk is measured between its first minima, so yes, some extra pixel resolution below this diameter can be helpful, but after you put more than three pixels in each dimension you will barely gain extra information from higher pixel density. As you stated it: F/16 will be ok on full frame, but F/32 will bring visible loss of detail. The whole "myth of diffraction" boils down to "diffraction hurts, but later than many believe" and is therefore no myth at all, although Sigma evidently wants us believe so :P

Again, your not understanding the point of my article. I'd been asked on several occasions about why someone would choose a sensor with smaller pixels, "because wouldn't diffraction just make the IQ worse when it affects the image at f/6.3 rather than f/8?" THAT is the myth I was aiming to debunk...that because diffraction STARTS affecting IQ on a sensor with smaller pixels at wider apertures than sensors with larger pixels, supposedly using smaller pixels is only useful if you use wider apertures. I wrote the article to explain to those people that diffraction is absolute, it exists due to the nature of light as it passes through the lens, and that pixels size really has nothing to do with it...diffraction is a lens trait. Sensor pixel size simply allows the ever-present effects of diffraction to be realized at a finer resolution when they are smaller.

Whether you have big pixels or small pixels, diffraction is going to affect the real image projected by the lens onto the sensor the same way. The exception is that smaller pixels will always be able to resolve more detail when more detail can be resolved (i.e. up through that first minima, which obviously grows as the aperture is stopped down.)  The point is that smaller pixels can never be a bad thing, but they can be a good thing, as far as image resolution is concerned.

Your reading something into my article that simply isn't there if your trying to make some argument about Sigma's geometric MTFs based on anything I've written there. Please don't twist my words. In assuming purely geometric traits for their lens MTFs, Sigma is really just looking for a way to edge their MTF plots higher up the chart, make their lenses seem better. Unsuspecting customers who really don't know what an MTF is will inevitably be comparing geometric MTFs of Sigma lenses with diffraction MTFs of Canon lenses, for example, and the comparison will be invalid. It's a cheat. Not exactly unexpected, someone was bound to try it sooner or later.

We (humanity) have known for a very long time that lenses do not behave purely geometrically, that they exhibit diffraction limited behavior when aberrations are eliminated.

1191
EOS Bodies / Re: Will Canon Answer the D4s? [CR2]
« on: February 06, 2014, 08:00:22 AM »
Wow, I'm enjoying the fireworks here!  ;D

Anyway, back to the topic on hand. I have no doubt Canon will answer the D4s (which is actually a rather minor update based on the rumored specs). I am more curious if Canon has a high mp, high DR answer in a moderately sized body like the D800. :)

HOW will they answer the D4s, though? I mean, is the 1D X not already THE answer? I find it highly doubtful that Canon would design, build, test, and release a successor to the wildly successful 1D X, which is already a far superior tool, just because of the minor updates coming in the D4s.

That's almost like a car manufacturer building an entirely new car only a few months after they put it on the market because their rival put prettier rims on their model, when the first car manufacturer already had kick-ass rims and was already selling the car like hotcakes.

1192
Third Party Manufacturers / Re: Sigma to Add Second MTF Chart for Lenses
« on: February 06, 2014, 07:54:37 AM »
Why can't I help but think that if Canon made this announcement there would be much celebrating in CR?

There wouldn't be. Geometric MTF is kind of a cheat.

While I do agree with you on that point, I ran across some very odd blog entry where someone claimed that diffraction blur won't affect final image quality. This blog entry claims that sharpening can be done and would therefore fully justify geometric MTF graphs as highly relevant, but it seems to ignore the increase in noise or other image defects. But maybe this happens only with inferior third party glass, you never know ...

LOL. Sorry, but you entirely misunderstood the point of my blog article, which had to do with the myth of diffraction as it relates to pixel size, a myth that presumes once you stop a lens down to the diffraction limited resolution of the sensor, you suddenly experience worse IQ than a sensor with larger pixels (yes, many photographers actually DO believe that). That's a different issue, though.

As for sharpening, it mitigates the impact of diffraction, it does not eliminate the effects of diffraction entirely, or make lenses behave purely geometrically. Sharpening an f/22 image does not make it diffraction limited f/2 performance. There are also limits as to how far sharpening takes you the farther you stop down...sharpening an f/32 or f/45 or f/64 is certainly not going to reduce the impact of diffraction enough to produce geometric results. It does, however reduce the muddiness of diffraction blurring that affects the f/16+ image to an acceptable level. But that's all post-processing. Lenses behave as lenses behave. Anything you do in post does not actually change the behavior of the lens.

1193
Animal Kingdom / Re: Show your Bird Portraits
« on: February 05, 2014, 09:00:29 PM »
GREAT stuff recently, guys!!

Love the nuthatch photos. Plenty of those in the mountains, but I don't get any in my back yard. Love their quirky antics.

The spotted twohee photo is wonderful. Love the composition and overall IQ. Excellent!

Jack, your really starting to get a hang of your setups now! Backgrounds look much better. Setups are getting better. I still think your underexposing just a bit...try a little bit more ETTR, which might help you extract a little more contrast in post. Looking forward to seeing how your setup stuff progresses. :)

1194
    I owned a Sony A7 for awhile but sold it and now have a 5DmkIII on the way. I found the Sony a joy to use but for several reasons I chose the Canon instead. For the shooter who does video as much as he does photographs, the 5DmkIII is still the best game in town.

    Here are a few reasons I dumped the Sony...
    • Lacks third party support, ie apps, software, and hardware devices like intravelometers, shutter release controls, etc.
    • Poor lens selection
    • Lacks some pro level features, ie dual SD cards

    The Sony as nice as it is, lacks some features I like. Maybe time will remedy some of the issues I had. [/list]

    How is Sony's software support overall? You mention a lack of...but what things do support it (if anything outside of ACR/LR)?

    1195
    EOS Bodies / Re: Will Canon Answer the D4s? [CR2]
    « on: February 05, 2014, 04:36:27 PM »
    The 6D further improved on the 5D2, yet you don't give it any respect.  You're biased, that's all.

    Sure, it further improved the IQ a bit, which was already excellent on the 5DII.  It improved the metering, too.  It didn't significantly improve the AF or frame rate, which were the 5DII's biggest deficits.  The 6D has a less robust shutter with a 1-stop lower max speed, slower Xsync, and a shorter rated lifespan. The 6D has a substantially longer shutter lag.  So considering IQ only, the 6D improved on the 5DII, but overall it's a mixed bag.  The 5DIII improved on the 5DII in pretty much every way.  At least on Amazon (not that it means much) the 5DIII is outselling the 6D.

    The 6D's biggest 'feature' is its lower cost. 

    Of course, AvTvM might say the 6D is not 'dated' because it has WiFi.  Nice if you want to upload your JPGs to Facebook on the fly, I suppose.  I thought it would be great for remote triggering, but someone pointed out that after a short time the connection drops, and you have to physically access the 6D to reactivate the link - that severely limits the utility, IMO (the WFT options for other bodies aren't limited in that way, but you pay a big premium for them).

    I consider
    * 5D III = 5D IIN
    * 6D = 5D II v1.1
     :D

    6D basically is a FF digital rebel to me. Marekting crippled product with a reasonable sensor and Wifi. About equally bad as Nikon D610, but better than D600, since it did not do splatter movies with its mirror. :-)

    LOL. OMG this is such an unmitigated amount of bias against the 5D III. The 1D IIn was a MINOR update to the 1D II. The 1D IIn was mostly the same, with literally the same sensor, af system, same digic, etc. The ONLY changes with the n were firmware...picture styles...and a better LCD screen. You do realize that, right?

    Trying to make it seem as though the 5D III is basically a 5D IIn is exceptionally naive and ignores a hell of a lot of facts. The 5D III was a massive upgrade compared to the 1D II -> 1D IIn update. It got an entirely NEW sensor that offered significant improvements in IQ (especially at high ISO), it got a new DIGIC chip, it got a radical update in AF system, it was the first non-1D body to get f/8 AF, it got a significant upgrade in metering sensor, it got the much-needed ergonomic and button placement upgrade, it got a weather sealing upgrade, it got a massive firmware update (akin to the 1D X firmware, which is WORLDS better than what the 5D II had), and a hell of a lot more!!!!

    Saying the 5D III is like a 5D IIn is completely ignoring ALL of the facts. Man, you know, you indicated in a response to me on another thread not long ago that you wanted me to show you some more respect. I'm happy to do so...but AvTvM...you really gotta DESERVE it. Saying crap like the 5D III is just a 5D IIn doesn't help, at all, in the respect department. It isn't as bad as the spinhappy tiraid against Canon as a greedy, selfish, and dumb company that is missing all it's opportunities (as if you actually know anything about it), but it's still dishonest. It speaks to a considerable level of either naiveté (i.e. you really just DON'T know what your talking about), or you do know what your talking about and it speaks to someone who is trying to pull a fast one on unsuspecting readers. Neither are very respectable.

    I can't respect the way you try to twist and convolute the facts. Sorry, but seriously...if you want to make a viable, cogent argument that other readers here on the forum will respect, don't go around making bullsh*t claims like "The 5D III is a minor upgrade, barely qualifies as a 5D IIn." That's a bold faced LIE! I KNOW you know that! And we aren't talking about a difference of opinion here...were talking about concrete facts. Verifiable, measurable differences in relative terms. The 5D III is not akin to a 5D IIn based on the actual, real-world, factual differences between a 1D IIn and the 1D II. The latter was a pitiful update, involving minor firmware and very minor hardware changes. The former was a very significant, measurably meaningful update involving massive changes in both firmware and hardware.

    Facts, bud. I'll respect you if you stick to the facts.  :-\

    1196
    I wonder if this is Sony's new plan of action. They've struggled with their electronics division for over a decade now, and it hasn't been profitable. I guess attaching themselves like a leech to Canon's lens lineup is the best way for them to make a sale.

    I have to admit, with Canon taking so long on a high MP, high DR body, I'm intrigued by the Sony A7r for landscapes myself. In most of my work, AF and frame rate are critical, but for landscapes, hell, all you really need is a high resolution sensor, live view, and a manual focus lens. Not much in the way of advanced technological requirements for landscape photography.

    Camera bodies last for a few years, then you replace them. It sounds like a Canon Big MP/High DR body is still a couple years off at least. By the time it actually hits (assuming Canon actually solves the low ISO DR issue), the A7r will be ripe for replacement anyway...

    One could even buy Zeiss lenses for the EF mount, and use them with the A7r EF adapter, and preserve the longevity of those (rather expensive) lenses in the event that Canon DOES eventually come out with a compelling (and affordable) high MP part in the future.

    1197
    Portrait / Re: We, The Photographers......Self Portaits..a Who's Who on CR
    « on: February 05, 2014, 04:17:44 PM »
     ;D LOL  ;D

    I find it extremely telling that every photo so far posted is of a guy!  :o In the few cases where a woman was in the frame, she clearly wasn't the photographer. (Although the one exception was the "portrait" of a foot...intriguing, that one.)

    I guess us guys are just WAAAY more obsessive about our technical hobbies than women. :P

    1198
    EOS Bodies / Re: Astrophotography with Canon60da
    « on: February 05, 2014, 03:55:37 PM »
    Mjayadev, all modern DSLR cameras use IR cutoff filters that are, in the grand scheme of things, quite cheap. Even the 60Da, which improves Hydrogen-alpha transmission, still barely achieves 70% transmission at that band (656nm). Most standard DSLRs have an IR cutoff filter with a long shoulder (or heel, rather) that gradually reduces deep reds and infrared transmission up through around 1100nm. Beyond 750nm, transmission of the near-IR band is low enough that it is largely inconsequential.

    With the 1D X, I would expect Ha (656nm) transmission to be somewhat poor. It won't really be any better or worse than other DSLRs though, however it is not as good as the 60Da.

    That said, should you worry? It ultimately depends on what you want to do. Are you just interested in photographing ultra wide field milky way images with landscape foregrounds? If so, don't EVEN worry yourself about the IR cut filter. :P If you want to do standard wide field imaging (anywhere from a few degrees to tens of degrees of sky), and your primary goal is full color imaging, I still wouldn't worry about Ha. Your images will be colorful, but not necessarily "realistic" as they will lack richness those deep red tones (which include Ha, S-II (Sulfur II) @ 645nm, and N-I & N-II (Nitrogen I & II) @ ~650nm and 658nm). You could also do narrow field astrophotography for objects less than a few degrees in size, but again, you would lack the richness of those deeper red tones.

    Overall, if you are generally just starting out with astrophotography, there are probably many more things you should be concerned with than what bands your sensor is sensitive to. There is a lot of technique you will need to learn in order to image the night sky effectively, especially if you are doing wide and/or narrow field imaging of DSOs. For any of these objects, you will need long "total integration time", which basically means total exposure time, via stacking multiple images, in the realm of 10 minutes to many hours long (and for some objects, exposures might need to span multiple nights!)

    In order to achieve that, you will need a tracking mount. More specifically, you'll need a german equatorial tracking mount. Tracking mounts come in a very broad range of costs and capabilities, ranging anywhere from a few hundred bucks for a basic mount capable of holding up to around 20lb, and capable of tracking up to a few minutes at most; through midrange mounts costing anywhere from a thousand to several thousand capable of holding up to around 40-60lb and capable of tracking up to five minutes, and maybe 10 minutes with very careful polar alignment and the assistance of an autoguiding setup; through high end mounts costing tens of thousands capable of holding anywhere from 45lb up to many hundreds of pounds, and capable of tracking anywhere from 10 minutes through many tens of minutes (30 minutes and longer) with exctremely high precision.

    The minimum recommended type of mount for entry level and midrange astrophotography are the Celestron CGEM & CGEM DX, Orion Atlas EQ-G, Skywatcher EQ-6, and iOptron iEQ, ZEQ, and CEM lines. These cost in the range of $1400 through $3500, and when properly aligned with the actual celestial pole (north or south) will get you fairly good tracking up to around 5 minutes (300 seconds). With the added support of a guiding scope and automatic guide camera hooked into a laptop computer, this can get you up to maybe 10 minute (600 second) exposures. This is a far more important aspect of astrophotography, especially early on, than whether your IR cut filter transmits enough of the deep red bands.

    To get good color fidelity, luminance detail, and maintain sharp (vs. oblong or trailed) stars, you will need tracking good enough to support a minimum of 4 minute (240 second) exposures, and a maximum of 10 minute (600 second) exposures. A good CGEM or Atlas mount and autoguiding is pretty essential for that. Once you have all of that taken care of, then you can start to concern yourself about other things, like emission bands and maybe even LRGB or narrow band (SII, Ha, OIII) filtration and mapped color (especially if you live in a city, where  light pollution is going to limit your exposures and reduce your detail anyway...narrow band imaging can be a godsend, albeit a more complicated and meticulous form of astrophotography.)

    Achieving exposures longer than around four minutes requires very good skill with polar aligning your mount, and with drift aligning your scope. Polar alignment can take some time to get precise, although setup routines in the midrange CGEM, Atlas, EQ6 and iOptron mounts simplify it a bit. Drift alignment is more important, in that it reduces declination drift. Equatorial mounts track stars across the sky, without field rotation, by tracking in right ascension. If the declination axis is not aligned properly, stars will "drift in declination", and over time that will result in oblong stars and soft nebula detail. Declination drift requires some good skill to refine to the point where you can expose for more than a few minutes.

    Even once you have aligned your telescope for smooth declination when tracking, periodic errors in the mount's gearing will still introduce some periodic wobble that will again reduce the accuracy of how stars are imaged. This is where autoguiding comes into play...autoguiding uses a small webcam like video sensor attached to a separate small guiding scope, along with some software (like PHD, or Push Here Dummy) to lock onto a star and make sure it tracks well. When a star's tracking deviates from where it should track, the the autoguider will instruct the mount to either speed up or slow down in RA, or additionally correct in Dec, to maintain tracking accuracy. This can get you up to 10 minute exposures when you have everything precisely aligned.

    Finally, exposing the night sky to good, rich color and low noise usually requires very long total exposure times. For deep sky objects (nebula, galaxies, clusters, etc.) you will usually need to expose multiple 4-10 minute exposures of the same exact region of the sky while the telescope tracks. You'll probably also need to produce dark frames and bias frames. All of this is then registered, calibrated, and stacked in a tool like DeepSkyStacker, to produce a final "integrated" image that has much less noise and much richer detail and color fidelity, than one single exposure.

    1199
    Third Party Manufacturers / Re: Sigma to Add Second MTF Chart for Lenses
    « on: February 05, 2014, 03:35:04 PM »
    Thing is, then what is the announcement about? What additional MTF are they going to publish?

    They're publishing a geometric MTF in addition to the diffraction MTF that they already publish.  The diffraction MTF takes diffraction of light into account, in addition to aberrations.  Diffraction limits the maximum resolution of lenses as the aperture gets smaller.  A geometric MTF ignores the effect of diffraction - in practice that means the f/8 lines will move up higher on the chart (higher is better).

    So which type does Canon publish for their lenses?

    Canon's are diffraction MTFs (I'm pretty sure everyone's are, actually, although that may not have been the case before computer-based lens design). 

    Like most manufacturers, Canon's are theoretical MTFs generated from mathematical modeling of the optical forumla of the lens.  Although Sigma apparently empirically measures the MTF of all their production lenses as part of QC, I presume their published MTF curves for each lens are theoretical.  Note that Sigma is wrong when they state, "Although Geometric MTF data is easier to measure and calculate…" – easier to calculate, yes, but you can't actually measure a geometical MTF, becuase real light passed through a real lens to measure MTF is subject to diffraction (they're also gramatically incorrect, data are plural  :P ).  With modern (even not that modern) computers, a diffraction MTF is easy to calculate once the optical design is known. 

    AFAIK, only Zeiss publishes empirically measured MTFs of real lenses.

    I'd call model-generated MTFs the "Maximum Potential" of a lens. Your average lens test, as I've mathematically demonstrated so many times before, are sensor-bound, and as see significant increases in resolving power, they see practically no improvement on a standard lens test performed with actual cameras, because the camera's spatial resolution becomes the upper limit. Two lenses may be WORLDS apart in theoretical (maximum potential) MTF, however when tested on a 22.3mp and a 36.3mp sensor, they show only marginal differences.

    So there is still a lot of value in having these theoretical MTFs published. They may "only" be generated mathematically based on a computer model of the lens materials and design, but they are certainly more accurate than a standard lens test.

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    Third Party Manufacturers / Re: Sigma to Add Second MTF Chart for Lenses
    « on: February 05, 2014, 03:30:17 PM »
    Why can't I help but think that if Canon made this announcement there would be much celebrating in CR?

    There wouldn't be. Geometric MTF is kind of a cheat. Besides, Canon lenses are already phenomenal with diffraction MTF. That's one of the biggest reasons Canon fans stick with Canon...their glass is second to none in the majority of cases.

    Canon's only real weak spot is the wider angle lenses and wide angle zooms. However, seeing as 2014 is supposed to be the "Year of the Lens", I suspect a lot of those problems will be remedied. The real big one, though, is the 16-35. That lens really needs an upgrade with better corner performance. A 14-24 with Nikon-level MTF would be extremely welcome. ;)

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